The eye gathers light from the environment and transmits the neural signals to the brain that result in vision. In most animals, the brain sends fibers back to the eye. The purpose of these returning efferent fibers is not well known in most species, especially mammals, because of the complexity of the visual system as a whole and the difficulty of direct experimental examination of efferent effects. Experiments on the horseshoe crab, Limulus polyphemus, have revealed a wealth of information about how the signals returning from the brain via the efferent fibers affect visual processing in the eye itself. In Limulus, a circadian clock in the brain drives the efferent fibers at night and is silent during the day. Efferent inputs modulate the structure and function of the retina in phase with the circadian clock and prime processes such as the daily shedding of phototransducdve membrane. Indeed, most of the major retinal processes appear to be modulated or controlled in some way by efferent inputs from the brain. This project investigates how the effects of efferent activity are realized at the cellular level in the retina. Specific aims are: 1) Directly investigate the effectiveness of octopamine in mediating structural changes caused by retinal efferents. 2) Investigate the cytoskeletal bases for structural changes that occur in response to efferent activity and light. 3) Test the photostasis hypothesis - is quantum catch homeostatically controlled and is such control a function of phototransductive membrane turnover? Our methodology emphasizes anatomical and physiological procedures including freeze fracture and deep etch ultrastructural studies, morphometric and stereological studies, video-enhanced-contrast, differential interference contrast microscopy and electroretinogram recordings. Our primary tenet is that comprehensive study of retinal cellular processes and their efferent control will yield additional fundamental understanding about the mechanisms that maintain visual sensitivity in the visual systems of animals including man.